Field of the Invention
The present invention relates to a transmission that transmits a motive power of an engine.
Description of the Related Art
A conventional transmission of an engine is provided with a first gear train, a fourth gear train, a third gear train, a sixth gear train, a fifth gear train and a second gear train arranged in this order from the side of a primary driven gear. On a drive shaft, jaw clutches are provided between a first driven gear and a fourth driven gear, between a third driven gear and a sixth driven gear, and between a fifth driven gear and a second driven gear. It is to be noted that the term “jaw (clutch)” used herein may be substituted with “dog (clutch)” in the same meaning.
Gear trains with lower gear ratios (closer to the first gear train) are subjected to higher tooth surface loads than gear trains with higher gear ratios (closer to the sixth gear train). Therefore, a counter shaft and the drive shaft that support the gear trains are subjected to higher loads at sections that support gear trains with lower gear ratios (closer to the first gear train) than sections that support gear trains with higher gear ratios (closer to the sixth gear train), for example, as described in Japanese Patent Laid-Open No. 2012-250660 (Patent Document 1).
With the conventional transmission mentioned above, the third gear train with a relatively low gear ratio is supported on center sections of the counter shaft and the drive shaft. Therefore, during a time of power (motive power) transmission by the third gear train, the counter shaft and the drive shaft are significantly deformed, which is unfavorable from the viewpoint of durability of the shafts.
As like as the tooth surface load on the gears, the load applied to the jaw clutches that transmits the motive power from the gear trains to the drive shaft are also higher at gear trains with lower gear ratios (closer to the first gear train) than at gear trains with higher gear ratios (closer to the sixth gear train). Therefore, jaw clutches that transmit the motive power from gear trains with lower gear ratios to the drive shaft have to have larger outer diameters.
In the conventional transmission, since the jaw clutches are disposed between the first gear train and the fourth gear trains, between the third gear train and the sixth gear train, and between the fifth gear train and the second gear train, and the gear ratio difference between the gear trains on both the sides of each jaw clutch is three steps, there is a corresponding (three-step) difference in outer diameter between the sides of each jaw clutch.
A fork groove into which a shift fork is to be fitted is formed in a center section of the jaw clutch. The fork groove needs to have a depth that provides a sufficient contact area between the jaw clutch and the shift fork at the side with the smaller outer diameter (on the side of the gear train with the higher gear ratio). This depth of the fork groove is the dimensional difference between the top of the side with the smaller outer diameter and the bottom of the fork groove.
However, at the side with the larger outer diameter (on the side of the gear train with the lower gear ratio), the dimensional difference between the top of the side of the fork groove and the bottom of the fork groove is larger than the dimensional difference on the side of the gear train with the lower gear ratio, or in other words, the fork groove has a larger depth. Therefore, the thickness of the section of the jaw clutch that supports the protruding jaw at the root thereof is reduced, which is unfavorable from the viewpoint of strength and durability.